Aerial photogrammetry may include an operation to generate an orthographic image on the basis of aerial photographs taken by a flying object. An example may be found at the homepage of the Geographical Survey Institute (URL: http://www.gsi.go.jp/gazochosa/gazochosa40002.html). Details of the orthographic image are disclosed in, for example, Japanese Patents Nos. 4138145, 4237868, and 4313462.
An orthographic image is generated on the basis of aerial photographs that are multiple aerial photographed images consecutively taken during a flight. The aerial photographed images are consecutively taken at photographic timings set so that the aerial photographed images taken in a consecutive manner or at very short time intervals will contain overlapping portions. Information of pixels constituting an orthographic image is mixed information of pixels acquired from the greatest possible number of the aerial photographed images. The mixed information of pixels is normally obtained by alpha blending. Multiple aerial photographed images are used in order to average the pixel information, compensate for missing image information, and correct imprecise image information.
Objects to be photographed by aerial photographing may include three-dimensional structures such as a cliff, a slope, a building, and a bridge. These objects tend to have parts that are hidden and thereby cannot be photographed, depending on point of view or photographing position. This generation of parts that cannot be photographed depending on point of view, is called “occlusion”. For example, depending on the point of view, it may not be possible to photograph a part of a side surface of a building that is occluded.
The photographic images to be used for aerial photogrammetry are taken during a flight. Thus, multiple photographed images containing overlapping portions may include both a photographed image containing a specific portion that is occluded and a photographed image containing the specific portion that is not occluded. In such cases, in a process of generating an orthographic image, image information of the specific portion that is occluded in one photographed image and image information of the specific portion that is not occluded in another photographed image are mixed, and as a result, the specific portion eventually has unclear image information and has a condition different from its actual condition. This inconvenience can cause a problem in generating an orthographic image containing a three-dimensional object such as a cliff, a slope, a high-rise building, or a bridge.